1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high-peak-power pulsed lasers used for the laser peening of solid materials and, more particularly, to methods and apparatus to control the application of the overlays applied for laser peening (also called laser shock peening, laser shock processing, and shock processing). The invention is especially useful for reducing the variability of the thickness of the overlays applied to the surface of the workpiece being laser shock peened.
2. Description of the Related Art
Old methods for shock processing of solid materials typically involve the use of high explosive materials or high-pressure gases, which are used to accelerate a plate, that strikes the solid material to produce shock waves therein. Such methods have several disadvantages. For example: (a) it is difficult and costly to shock process non-planar surfaces and complicated geometries, (b) storage and handling of the high explosive materials and high pressure gases pose a hazard, (c) the processes are difficult to automate and thus fail to meet some industrial needs and (d) high explosive materials and high pressure gases cannot be used in extreme environments such as high temperatures and high vacuum.
Shot peening is another widely known and accepted process for improving the fatigue, hardness, and corrosion resistance properties of materials by impact treatment of their surfaces. In shot peening, many small shot or beads are thrown at high speed against the surface of a material. The shot or beads sometimes escape from the treatment equipment and scatter in the surrounding area. Since the shot or beads might get into surrounding machinery and cause damage, shot peening usually cannot be used in a manufacturing line. Ordinarily such shot peening cannot be used on machined surfaces without a high likelihood of damaging them. In addition, shot peening has problems maintaining consistency of treatment caused by inherent wear of the shot by the shot peening equipment.
Laser peening equipment, however, can be incorporated into manufacturing lines without danger to surrounding equipment. Laser peening with coherent radiation has several advantages over what has been done before. For example, the source of the radiation is highly controllable and reproducible. The radiation is easily focused on preselected surface areas and the operating mode is easily changed. This allows flexibility in the desired shock (peening) pressure and careful control over the workpiece area to be laser peened. Workpieces immersed in hostile environments, such as high temperature and high vacuum can be shock processed. Additionally, it is easy to laser peen the workpiece repetitively. This is desirable where it is possible to enhance material properties in a stepwise fashion. Laser peening the workpiece several times at low pressures can avoid gross deformation, cracking, and spallation of the workpiece while nonplanar workpieces can be laser peened without the need of elaborate and costly shock focusing schemes.
Laser peening utilizes two overlays: a transparent overlay (usually water and hereinafter referred to as overlay water) and an energy absorbing opaque overlay; an oil based paint; an acrylic based black paint; or an energy-absorbing tape (and hereinafter referred to as paint). During processing, the paint is locally applied to the surface of the workpiece to be processed, followed by the application of the overlay water. A laser beam is directed to pass through the overlay water and is absorbed by the paint, causing a rapid vaporization of a thin layer of the paint and the generation of a high-amplitude shock wave in the workpiece. The shock wave cold works the surface of the workpiece and creates compressive residual stresses, which provide an increase in fatigue properties of the workpiece. A workpiece is typically processed by laser peening a matrix of overlapping spots that cover the fatigue critical zone of the workpiece.
The method of rapidly applying an energy absorbing overlay or paint, followed by a transparent overlay or overlay water, subsequently laser shock peening the workpiece, and then removing the overlay is presented in U.S. Pat. No. 5,471,559. This method has been successfully implemented into laser peening equipment and has dramatically decreased the processing time and cost; however, the variability in the thicknesses and/or uniformity of the paint or overlay water can potentially lead to differences in the resultant residual stresses in the workpiece and thus change the fatigue life capabilities of the laser-peened workpiece.
To provide a consistent laser shock peening process, the application of the paint and overlay water need to be monitored during processing and subsequently controlled to ensure that the thickness and/or uniformity of the paint and overlay water are the same for each laser shot. What is needed is a method to monitor and control the application of the paint and overlay water during processing to apply a consistent thickness and/or uniformity for each laser shot.
The present invention provides a method of laser peening that can be used in a production environment to significantly reduce the variability of laser peening by monitoring and controlling the thickness and/or uniformity of both the paint and the water overlays. The method includes the steps of painting the workpiece to be laser peened with a layer of paint applied to a small area about 2 to 3 times the diameter of the laser-peening spot, measuring the thickness and/or uniformity of the paint prior to the application of the overlay water, and recording these measured values. Subsequently, overlay water is applied, forming a thin, flowing layer over the previously painted portion, and the thickness and/or uniformity of the overlay water is measured at a specific time after its initial application, but prior to firing the laser, and the measured values are recorded. The thickness of the paint or overlay water is the distance between the exposed surface of the paint or overlay water and the surface upon which it is applied, and the uniformity is the difference in the thickness between two or more locations on the overlay. The measured values for the paint and water overlay are then compared to the specified range. The specified range may be a predetermined range based upon engineering judgment, or it may be determined by statistical methods. If the measured values are within the specified range, then the laser beam is directed through the water overlay and onto the painted area. If the measured values for the paint and water overlay thickness and/or uniformity are not within the specified range for the process, then the laser is not allowed to fire and the high-speed water jet is activated to wash the paint spot off of the workpiece. The automated process of applying paint and overlay water may be repeated without changes. Alternately, adjustments in the overlay-application parameters or applicator head positions may be made to change the thickness and/or uniformity of the overlays to bring them into the specified range. Adjustments may be made by the controlling computer that sends signals to electronically controlled valves to alter the flow rate, pressure, and application duration of the paint and overlay water applicators, to bring the thickness and/or uniformity values for the two overlays into the specified range for the process. After the adjustments are made to the overlay-application parameters or applicator head positions, the process sequence is restarted.
In the present application, the term xe2x80x9cspecified rangexe2x80x9d is the bounds that the thicknesses of the overlays need to be within for continued laser peening operations. xe2x80x9cSpecifiedxe2x80x9d in the broadest sense may mean limit values created with or without knowledge or historical data on successful laser peening. The term xe2x80x9cpreselected rangexe2x80x9d is a fixed range that has been calculated before laser peening commences. The term xe2x80x9cstatistically determined range of valuesxe2x80x9d is the limit ranges calculated xe2x80x9con the flyxe2x80x9d and between each laser peening operation based upon the same ongoing running total or historical data collected and analyzed. For example, the ranges could be calculated as xc2x1 one standard deviation from the mean of the last 100 thickness samples. This example is not limiting, other statistical methods and functions may be utilized to create operational bounds or limits to effect overlay thickness control.
The invention comprises, in one form thereof, a method of controlling the thickness of the paint and overlay water. The method involves applying the paint and then the overlay water to the workpiece to be processed based upon thickness. The paint is first applied to a portion of the workpiece. During the application of the paint overlay, the thickness is measured in real time. Upon reaching the specified range for the thickness, a signal is sent from the control computer to terminate the application of the paint and initiate the application of the overlay water. The thickness and/or uniformity of the overlay water are monitored, and when the measured values are within the specified range, the laser is fired and the laser beam is directed through the overlay water and onto the painted area.
The invention comprises, in one form thereof, a method of applying a continuous flow of overlay water to the surface of the workpiece during the laser shock peening process. The overlay water is continuously directed over the surface of the workpiece to be processed. The application of the overlay water is accomplished with an applicator in direct contact with the workpiece or a high-speed water jet. A jet of air is directed toward the overlay water near the area where the paint is to be applied to divert the overlay water to other areas on the workpiece. The air jet is maintained during the application of the paint. The thickness of the paint is measured and when it is within the specified range, the air jet is turned off to allow the overlay water to reform and flow over the paint. The thickness of the overlay water is measured and when it is within the specified range, the laser is fired and the laser beam is directed through the overlay water and onto the painted area.
The invention comprises, in one form thereof, a method of controlling the thickness of the energy absorbing overlay with a tamping device such as an air jet that is directed to the energy absorbing overlay. The position and operation of the air jet may be determined for each workpiece through trial and error experimentation; or preferably, the position and operation of the air jet may be automated. If the air jet is automated, the position and operational parameters of the air jet may be controlled in real-time or may be established through a calibration sequence prior to processing workpieces. In an automated process, the thickness of the energy absorbing overlay is monitored during or after its application to the surface of the workpiece. If the thickness is not within the specified range, the control computer triggers the operation of at least one tamping device such as a jet of air (or other suitable gas) directed toward the paint to conform it to the proper thickness. The air jet or jets may be turned off prior to the application of the water overlay. The tamping device may also be a mechanical tamping block that physically contacts the overlay.
The invention comprises, in a further form of the previous description, the use of air jets or nozzles to control the thickness of the water overlay.
The invention comprises, in yet another form thereof, an apparatus for measuring the thickness of the paint or overlay water that are used for improving properties of a workpiece by providing shock waves therein. The apparatus includes a device to measure the thickness of the paint or overlay water applied to the surface of the workpiece with overlay applicators. Means to control the flow of the overlay materials are provided. A tamping device operatively associated with the measurement device and control unit is used to change the thickness of the overlay on the surface of the workpiece. A laser peening system is operatively associated with the control device to provide a laser beam, which is directed through the overlay water to create a shock wave in the workpiece. The measurement device, material applicators, material applicator flow control means, tamping device and laser peening system, are connected to a control computer that controls the operation and timing of each of the applicators, measurement devices, control valves, tamping devices, and the laser.
The control computer collects the measured thickness values and compares these values to the specified values for the laser peening operation on the workpiece being processed. The control computer determines if the values for an overlay are within the specified tolerance range. If they are within the specified range, the laser operatively associated with the process is fired to generate a shock wave on the surface of the workpiece. If these measured values are not within the specified thickness range, the computer prevents the laser beam from being fired and sends a signal to remove the paint and overlay water. The control computer makes adjustments to the flow of the overlay applicator that is out of the specified range, reapplies the paint and overlay water, again measuring the thickness of the paint and overlay water at specific points during or after application. The control computer may also activate a tamping device such as an air jet to change the thickness or smooth the paint or overlay water.
An advantage of the present invention is that it provides a method to ensure a consistent thickness of an overlay, which provides more consistent and repeatable laser peening results within the workpiece.
Another advantage of the present invention is that the control computer can control the timing of the application of the paint and overlay water by measuring the thickness during application of these overlays.
Yet another advantage of the present invention is that the control computer can make adjustments to the control valves to modify the thickness of the paint and overlay water.
A further advantage of the present invention is that the control computer can modify the thickness of the overlays applied to the surface of the workpiece by use of an tamping device.